1. Field of the Invention
The present invention relates to apparatus for controlling the lateral positioning of elongated web material by means of fluidic medium flow between the web material and a bearing surface. More particularly, the present invention relates to apparatus for controlling the location of the edge of a movable elongated web material relative to a reference line through controlled pressurized gas introduction between the web material and a bearing surface. Although not necessarily limited thereto, the present invention is particularly useful for maintaining one side edge of a movable, elongated tape such as a magnetic tape against a reference edge as the tape is motivated lengthwise over an air bearing surface.
2. Description of the Prior Art
Various prior art devices use a fluidic medium and air in particular for supporting an elongated web material over a bearing surface. The problem of proper lateral positioning of this elongated web material over the bearing surface is particularly acute when the web material is a magnetic tape which must be guided over read/write heads for detecting magnetic indicia recorded on the tape. One prior art arrangement for providing the edge guiding result is to apply mechanical spring pressure to one side of the tape so as to direct the opposite side of the tape against a reference edge. U.S. Pat. No. 3,850,358 by Nettles shows an exemplary arrangement for this spring biased edge guiding. Yet another edge guiding arrangement known in the prior art employs jets for blowing pressurized air against one edge of the tape so as to literally push the other side of the tape against a reference edge.
Yet another approach in the prior art to the edge alignment problem is to employ differential air bearing flow beneath the tape so as to create a force unbalance and thus lateral movement of the tape against a reference edge. The prior art devices for effecting such a result have employed dual manifolds beneath a perforated bearing surface with separate pressure sources for each manifold and control systems to insure proper edge alignment. Examples of such dual manifold differential flow devices are shown in U.S. Pat. No. 3,893,176 by Jones and likewise in the IBM Technical Disclosure Bulletin articles entitled "Constant Fly Height, Constant Stiffness, Variable-Width Air Bearing" by Stevenson (Vol. 18, No. 11, April 1976, page 3567), "Tape Control Device" by Durbeck and Jones (Vol. 17, No. 4, September 1974, page 1188), "Steering Air Bearing With High-Speed Air Switching" by Jones and Wong (Vol. 15, No. 9, February 1973, page 2743) and "Tape Steering Device" by Jones and Patlach (Vol. 15, No. 9, February 1973, pages 2744-2745).
All of the prior art differential flow devices employ symmetrical hole patterns with dual manifolds or plenum chambers feeding separate sides of the aperture arrays and with the differential flow being established by controls of two separate pressure sources. FIG. 2 of the Jones and Patlach article entitled "Tape Steering Device" in the IBM Technical Disclosure Bulletin mentioned above employs a single pressure source and plenum chamber but further includes a positionable piston within the plenum chamber for controlled opening and closing of the bearing surface apertures so as to further control the lateral positioning of the web material as it passes over the bearing surface.
Although the aforementioned exemplary prior art is generally satisfactory in operation, practical implementations of these devices require acceptance of various disadvantages. For instance, the spring biased edge guiding creates additional loading on the edge of the magnetic tape and the differential flow devices using dual chambers requires multiple pressurized gas sources and relatively sophisticated controls for those sources.